1. Field of the Invention
This invention relates to a method and apparatus for introducing samples of a gas into a vacuum chamber. More particularly, this invention relates to a method and apparatus for introducing samples of a chemically reactive atmosphere at relatively high pressure into the low pressure chamber of a mass spectrometer.
2. Description of the Prior Art
Mass spectrometers are extensively utilized to provide information concerning the components of a gas mixture. A typical application is chemical vapor deposition, where gas samples from the process reactor are analyzed by a mass spectrometer, the information obtained thereby being used to control the partial pressures of active components in the process reactor. The technique is also used to monitor the progress of chemical reactions wherein the reactants are in a gaseous state.
The analyzer chamber of a mass spectrometer is maintained at sufficiently low pressure, usually below 10.sup.-4 Torr, to assure that ion separation is not hindered by collisons with gas molecules. It is often the case that the pressure in the process reactor exceeds the maximum permissible pressure in the mass spectrometer. For example sputtering, in which a source material is subjected to ion bombardment, usually takes place at pressures from 10.sup.-3 to the low 10.sup.-2 Torr range. Other examples of processes that are conducted at pressures beyond the operational range of mass spectrometers are chemical vapor deposition and etching processes. In such cases the mass spectrometer is provided with its own pumping system in order to maintain desired pressure conditions in its vacuum chamber.
As the gas samples being introduced from outside the vacuum chamber are at relatively high pressure, in order to achieve pressure reduction a component having a low gas flow conductance is interposed between the vacuum chamber and the process reactor. This is usually one or more very small orifices.
Many of the above mentioned processes employ highly reactive or corrosive gases that can attack the mass spectrometer. A longstanding problem in the analysis of such gases has been degraded performance of the mass spectrometer due to reaction of its components with these gases. Another problem in the art is the deposition or sorption of non-volatile reaction products on the surfaces of the spectrometer, leading to undesirable memory effects or signal drift. Such reaction products can even obstruct the orifice and thus completely invalidate the analysis. Orifice blocking is particularly severe in chemical vapor deposition processes that are performed at pressures exceeding 1 Torr, and rising as high as atmospheric pressure. The sampling orifice must be extremely small to accommodate such high pressures and is consequently highly vulnerable to physical obstruction by reaction products.
It has been attempted to retard the interaction of reactants with the components of the mass spectrometer by reducing gas flow into the vacuum chamber. When this is done by reducing the orifice size, blocking becomes more severe. Furthermore the small orifice can limit the signal developed from the sample and hence the sensitivity of the mass spectrometer. A more satisfactory approach uses staged pressure reduction, in which the vacuum chamber of the mass spectrometer is separated from the process reactor by a plurality of intervening chambers, each having a progressively lower pressure, and having larger orifices for fluid flow therebetween. This method, however, requires complex and expensive apparatus, and due to practical limitations on vacuum pump capacity, the gain in orifice size is limited.
Another approach is shown in French et al, U.S. Pat. No. 4,023,398, in which a gas curtain flows through a chamber disposed between an ionization region and a vacuum chamber. The curtain blocks gas flow through an orifice leading to a mass spectrometer. The chamber containing the curtain gas constitutes a dead space that limits the switching rate of the protective effect of the curtain. Furthermore the degree of blocking of the orifice by the curtain cannot easily be controlled. In practice this approach requires high curtain gas flows to provide effective protection, and the gas has to be separately pumped. As the curtain gas flow is high, only condensable cryopumped curtain gases are feasible in many mass spectrometric applications, as other commonly used pumping methods cannot easily provide high pumping rates in the small volume of a mass spectrometer housing. Furthermore in order not to affect the chemical process being carried out, it is desirable to minimize the volume of protective gas that is introduced to the reaction. The high curtain gas flows needed in the French approach directly conflicts with this requirement.